Process for transitioning

Abstract

Process for transitioning from the production of a first polymer to the production of a second polymer in a gas phase fluidized bed or stirred bed reactor by a) conducting a first polymerization reaction in the reactor using a first polymerization catalyst system to produce a first polymer, b) stopping the first polymerization reaction and removing at least the majority of the polymer in the reactor, c) introducing into the reactor a seed-bed containing at least 50 ppm of contaminants, in which the seedbed is maintained under at least 50 ppm but less than 500 ppm of contaminants during storage, and d) treating the seed-bed in the reactor to reduce the amount of contaminants. Step e) involves conducting a second polymerization reaction in the reactor to produce a second polymer.

The invention claimed is: 
     
       1. A process for transitioning from the production of a first polymer to the production of a second polymer in a gas phase fluidised bed or stirred bed reactor, said process comprising the steps of:
 a) Conducting a first polymerisation reaction in the reactor using a first polymerisation catalyst system to produce a first polymer, 
 b) Stopping the first polymerisation reaction and removing at least the majority of the polymer in the reactor, 
 c) Introducing into the reactor a seed-bed comprising at least 50 ppm of contaminants, wherein the seedbed has been maintained under at least 50 ppm but less than 500 ppm of contaminants during storage, 
 d) Treating the seed-bed in the reactor to reduce the amount of contaminants, and 
 e) Conducting a second polymerisation reaction in the reactor to produce a second polymer. 
 
     
     
       2. A process according to  claim 1  wherein step (b) comprises stopping the feed of the first catalyst system, and allowing the reaction to continue whilst the activity dies off before polymer removal is commenced. 
     
     
       3. A process according to  claim 2  wherein the removal of polymer is commenced whilst the reaction continues and/or before the feed of catalyst system is stopped. 
     
     
       4. A process according to  claim 1  wherein the amount of polymer remaining in the reactor after step (b) is reduced by at least 80 wt % compared to the average amount of polymer in the reactor during steady-state production of the first polymer. 
     
     
       5. A process according to  claim 1  wherein prior to step (c) one or more steps are performed to treat the reactor, said steps comprising at least one of (i) use of a deactivating agent to deactivate any remaining traces of the first polymerisation catalyst system, and (ii) purging to remove any treatment agents which have been added. 
     
     
       6. A process according to  claim 1  wherein the seedbed comprises greater than 55 ppm of contaminants and/or wherein the seed bed comprises less than 250 ppm of contaminants. 
     
     
       7. A process according to  claim 1  wherein the contaminants comprise one or more of water, carbon monoxide and carbon dioxide and/or wherein in step (d) the amount of contaminants in the seed bed in the reactor is reduced to less than 20. 
     
     
       8. A process according to  claim 1  wherein the reactor is maintained substantially closed both throughout the bed removal, the seedbed introduction and the transition. 
     
     
       9. A process according to  claim 1  wherein the reactor is a gas phase fluidised bed reactor. 
     
     
       10. A process according to  claim 6  wherein the seedbed comprises less than 100 ppm of contaminants. 
     
     
       11. A process according to  claim 6  wherein the contaminants comprise one or more of water, carbon monoxide and carbon dioxide. 
     
     
       12. A process according to  claim 6  wherein in step (d) the amount of contaminants in the seedbed in the reactor is reduced to less than 20 ppm. 
     
     
       13. A process according to  claim 6  wherein the reactor is maintained substantially closed both throughout the bed removal, the seedbed introduction and the transition. 
     
     
       14. A process according to  claim 6  wherein the reactor is a gas phase fluidised bed reactor. 
     
     
       15. A process according to  claim 7  wherein in step (d) the amount of contaminants in the seedbed in the reactor is reduced to less than 10 ppm. 
     
     
       16. A process according to  claim 7  wherein the reactor is maintained substantially closed both throughout the bed removal, the seedbed introduction and the transition. 
     
     
       17. A process according to  claim 7  wherein the reactor is a gas phase fluidised bed reactor. 
     
     
       18. A process according to  claim 4  wherein the amount of polymer remaining in the reactor after step (b) is reduced by at least 95 wt % compared to the average amount of polymer in the reactor during steady-state production of the first polymer. 
     
     
       19. A process according to  claim 6  wherein the seedbed comprises at least 75 ppm of contaminants. 
     
     
       20. A process according to  claim 12  wherein in step (d) the amount of contaminants in the seedbed in the reactor is reduced to less than 10 ppm.